The present invention relates to a method of manufacturing a laminated thermoforming which consists of a thermoformable thermoplastic resin, resin mixtures, or a resin matrix as substrate material of the thermoforming and at least one deformable backing material.
The present known methods in which laminating is effected in particular in order to improve the optical appearance or the thermal insulation and in which foils, non-wovens, textiles or foams are used as backing material comprise, inter alia, the subsequent backing of a substrate part with the use of additional adhesive, direct back spraying in the low-pressure method, direct back embossing in the compression-injection method, direct back-foaming in the foaming method and thermoforming.
Further methods which are known and possible at the present time in which metallic and/or electric conductive structures are applied to or in the plastic, in particular in order to improve or produce electromagnetic screening are, among others, the subsequent integrating of a metal fabric by special constructions in the entire part, by the working of conductive polymers in the injection molding process, by coating with conductive varnishes, by lining with metallic foils, by vapor deposition with aluminum, or by electro-chemical/galvanotechnical coating.
Furthermore, a screening material capable of deep drawing is available on the market as multiple-sandwich material of EVA non-woven fiber and metal non-woven fiber especially for EMI screening, the EVA non-woven fiber serving as thermoplastic adhesive and the metal non-woven fabric consisting of an alloy which melts upon the processing.
From German Utility Model G 92 16 080.8, a deep-drawing part of thermodeformable thermoplastic resin, resin mixture, or a resin matrix is known which consists of a fiber-reinforced polypropylene or polyethylene or a corresponding copolymer.
By the properties described in said utility model, this deep-drawing part finds many fields of use, including ones in which, in addition to the technical function, optical and decorative requirements and/or noise-dampening, insulating, electrical, electromagnetic or stability requirements exist which cannot be satisfied without additional surface treatment.
From German Patent Application DE 42 11 077 a method is known for the production of a molding having a structured surface in which a decoration substrate is first of all inserted into a deep-drawing mold, a plastic plate is introduced for the drawing of a molding, heated to forming temperature and then pulled into the mold by vacuum in the region of the deep-drawing mold. In this connection, the side facing the plastic molding should be at least partially material-locked in a surface layer of the plastic molding.
This method has the disadvantage that this material-locked connection cannot be obtained to a sufficient extent in all cases of use. In particular, in the event of larger deformations such as deep troughs and difficult geometry of the molded parts, such as undercuts, the required shaping and/or connection cannot be reliably obtained. Furthermore, the known method cannot be reliably used in the case of decoration materials of little or no air permeability since uncontrollable inclusions of air can remain between the plastic panel and the decorative material.
It is an object of the present invention to provide an inexpensive method for the simple manufacture of thermoformed parts which are backed on at least one side and consist of heat-deformable thermoplastic resin material, in which a dependable connection is obtained between the deformed plastic panel and the backing web and which is suitable both for backing materials for decorative or insulating purposes as well as materials of metal or other high-strength materials.
This object is achieved by the invention in the manner that before the thermoforming, at least one deformable backing web is introduced in the space between the plastic panel and a deep-drawing mold, the deformable backing web consisting of a material which stretches under the conditions of pressure and temperature which occur upon the thermoforming and/or of a material which does not stretch even under the pressure and temperature conditions occurring upon the thermoforming but is deformable on the surface by its intermeshing or change in angular position, and that the pressure and temperature conditions of the thermoforming process are so adjusted that plastic panel and backing web are permanently attached to each other by incorporation of melting together.
The invention proceeds in this connection from the discovery that a distinction may basically be made between two types of material for the backing web which is to be applied.
On the one hand, those materials which under the pressure and temperature conditions occurring during the thermoforming are themselves extendable and stretchable, in which connection the possible limits of the pressure and temperature conditions of both the material of the substrate panel and the backing material can be determined.
Other materials such as, for instance, metals, including metal alloys, or glass or carbon fibers, are as a rule extendable to stretchable only at temperatures which are above the limits of the processing temperature of a substrate plate. For the use of such materials, extendibility or stretchability of the material itself is, however, not necessarily required, inasmuch as backing webs for other uses which are deformable by their surface structure are available or can be produced. There are concerned here, in particular, metal-wire fabrics which are extendable in the surface due to their mesh shape, and metal-wire webs in which the wires are at a predetermined angle to each other and surface stretchability is possible during the processing by change in the angles.
Both types of material can also be present in mixed form, for instance as mixed fabric of cotton having metal wires woven or knitted therein.
Both materials, as well as mixed forms, can be undetachably attached to the substrate panel by the method of the invention if the pressure and temperature conditions of the thermoforming process are so selected that the backing web is incorporated into the substrate panel or melted together with it, in which connection the thermoforming process is preferably deep drawing.
By additional support by compressed air, a better precision in the shaping of the sandwich of plastic panel and backing web is obtained which makes undercuts and 90xc2x0 bends possible since the sandwich is pressed with considerably greater forces into the deep-drawing mold. This goes hand in hand with a more dependable anchoring of the backing web in the surface of the plastic panel as a result of these greater forces. Furthermore, higher stretching and elongation forces can be exerted on the material or the structure of the backing web, as a result of which the precision in shaping is also improved.
The reliability of the material-locked connection is increased also in the case of backing webs of little or no air permeability.
For backing materials of high permeability such as, for instance, thin textiles in which only a controlled depth of penetration into the surface of the plastic panel is to take place, the method of the invention is particularly suitable, particularly in the case of complex shaping. Depending on the materials used, the method of the invention makes it possible to vary and optimize the parameters, pressing pressure and temperature of the plastic panel, which are causal for the shaping, in particular, of complex mold regions, and for the depth of penetration within a widened range for the pressing pressure. Thus, with the same shapeability, by lower temperature together with higher pressing pressure, the penetration caused by the temperature-dependent flowability can be reduced.
The same is true here for backing materials of metal or other high-strength materials which do not melt together with the plastic panel but adhere by targeted degree of the incorporation.
Apparatus at the present time permit support by compressed air of up to 16 bar, in which connection, a compressed-air support of between 0.5 and 10 bar is preferably used, depending among other things on the size of the molding and the forces occurring thereby.
The method is suitable both for the complete, surface-covering backing of thermomoldings and for the backing of parts of the surface.
By the gentle application of the backing web upon the thermoforming in the pressure range of 0.5 bar to maximum about 10 bar (gauge pressure), excellent quality features, good retention of the soft textile character and good color constancy of the decorative material are obtained.
The above-mentioned possibilities can be supported, in a further development of the invention, in the manner that the backing layer is preheated at least on one side before the deep-drawing process to such an extent that a clear reduction in the modulus of elasticity takes place, i.e. a clearly improved stretchability and extendibility, but no other substantial properties of the material are changed. In other words, the heating should remain, particularly in the case of partially crystalline plastics, below the crystallite melting point (in the case of polypropylenes  less than 130-140xc2x0 C. and in the case of polyester  less than 220xc2x0 C.) in order still to assure sufficient constancy of color.
By this at least one-sided heating of the backing web preceding the thermoforming, the result can be obtained that the backing web only now becomes deformable, or its deformability improved so that the backing web rests against the deep-drawing mold without the forming of folds or cracks.
The method is suitable, in particular, for deep drawing with positive or negative mold, possibly with the support of the ejection by upper ram, known in the case of this process.
The method of the invention is particularly advantageous when there are selected for plastic panel and backing web combinations of materials which bind intimately to each other as a result of the pressing pressure which occurs upon the thermoforming, and particularly the deep-drawing process itself and/or the temperatures which occur in this connection. In this way, the application and use of adhesive materials is superfluous, which simplifies both the method of manufacture and the later recycling.
If a surface in the case of which such a combination of materials is not possible is to be produced with the backing web, a multi-ply or multi-layer backing web can also be selected on the side of which facing the plastic panel there is or will be applied a layer of material which binds with the plastic panel upon the deep drawing. In this way, the advantage described for the manufacturing process is retained and the connecting of the materials within the multi-layer backing web can be loosened by other processes upon its manufacture.
By a multi-layer backing web there is understood in this connection the use of an additional intermediate web consisting of a third material, which is particularly advisable when the materials of the plastic panel and of the backing web do not enter into a material-locked connection with each other or only one which is not sufficiently reliable, but both materials enter into such an attachment with the material of the intermediate web.
Independently of this use of an intermediate foil, its use may also be suitable in the case of a wide-mesh backing web in order to prevent a xe2x80x9cpenetratingxe2x80x9d of the material of the plastic panel through the meshes of the backing foil.
If none of the above-indicated possibilities of such combinations is possible due to considerations of cost or other reasons, the backing web can be provided on the side facing the plastic panel with an adhesive so as to obtain an attachment upon the thermoforming, particularly the deep-drawing process.
As materials for the backing layer there are suitable in this connection in particular foils, needled non-wovens, and textiles of all types, particularly of cotton, polyamide, polyester or polypropylene and furthermore TPE or PVC foam foils and cross-linked foam foils having a base of polyurethane, polyethylene or polypropylene. Furthermore, particle foams are suitable, for instance polypropylene particle foams.
The construction of the backing materials such as woven fabric, knitted fabric, machine knitted fabric, weight per unit surface, stretchability, etc. is dependent on the radii desired and the stretching of the material which occurs upon the deep drawing.
As substrate material for the backing there are particularly suitable plastic panels which have a high stiffness (high modulus of elasticity), for instance plastic panels of polypropylene reinforced by glass fibers, natural fibers or talcum, or styrol-acrylonitrile (SAN) reinforced with glass fibers.
For the manufacture of moldings, having increased properties of mechanical loadability and/or temperature resistance, there can advantageously be used backing webs of metal wires or threads or other high-strength materials, in connection with which a significantly better profile of properties can be obtained by suitable angular positions of the wires.
In the case of knitted articles of metal threads or other high-strength materials, the knitted fabric can preferably be made so flexible by suitable design of the stitches that the production of moldings of large three-dimensional deformation is possible.
In a further development of the method of the invention, the backing web consists of an electrically conductive material and is provided with at least one contact element which is accessible after the thermoforming.
The backing with an electrically conductive material makes possible the use of the molding both as electromagnetically screening element with at least one electric contact element for connection to the reference potential and as heatable molding with at least two contact elements, in which connection, depending on the use, a decorative and/or insulating backing can be applied in the same operation simultaneously with the electrically conductive backing as second further backing.
For at least three-layer moldings consisting of the plastic panel, an electrically conductive and in particular metal backing web, and a decorative backing web there are preferably selected combinations of materials which are intimately bonded by the pressing pressure which occurs by itself upon the thermoforming and/or the temperature occurring in this connection. In this way, the electrically conductive, particularly metal backing web is incorporated between two materials which melt together.
The backing web can, in particularly advantageous manner, be fed into the space between mold and plastic panel via a separate clamping frame or endlessly over a system of rollers.
In order to determine the position of the backing web with respect to the molding and prevent an uncontrolled xe2x80x9cdrawing-inxe2x80x9d into the mold, a mechanical fastening is effected of the backing web to the mold or the clamping frame, in which connection the fastening elements, which can be developed for instance as fixed or movably supported needle system which can have yieldability in the direction of the elongations occurring upon the thermoforming, in order to avoid excessive stretching of the material of the backing web upon the forming.
Furthermore, the backing web can be introduced in such a manner that, in the case of a positive mold, it already completely or partially surrounds the mold or in the case of a negative mold it extends entirely or partially into the inside of the mold.
Upon a backing of partial surfaces, corresponding fixing devices are to be provided on or in the mold itself, they permitting an application or insertion in register upon the manufacturing process and assuring a suitable fixing of the edges during the thermoforming process, or the fixing can be effected by separate frame which is also covered by the plastic panel upon the deep drawing and remains in the finished part.
In a special further development of the invention, a plastic panel which already has a backing on the side facing away from the mold is used to produce moldings which are backed on both sides. This can be advantageous, in particular, if the subsequent use of the thermal molding requires both a noise damping effect, which can be obtained by backing with a sound-damping material on one side, and a decorative appearance, which can be obtained by a backing on the other side.
With respect to suitability for recycling, it is particularly advantageous to use substantially the same plastic materials, in particular polypropylene, for substrate material (plastic panel) and backing web. In this way, it is no longer necessary to separate substrate and backing materials on the reprocessing.